This invention relates to an integrated printhead comprising an array of groups of ink jet elements in M rows and L columns wherein each group has a single row and column address, first addressing means PJ associated with the array of groups for selecting one of the M rows of the array of groups of ink jet elements in M rows and L columns, second addressing means XK associated with the array of groups for selecting one of the L columns of the array of groups of ink jet elements in M rows and L columns, and third addressing means AI associated with the array of groups for selecting one ink jet element in each group of ink jet elements.
In particular, this invention relates to a thermal ink jet head of the IDH type (Integrated Drive Head) wherein a plurality of thermal resistances or resistors are selectively activated by an external control circuit to produce the emission of droplets of ink through nozzles placed in correspondence with the resistors.
An IDH integrated printhead is known from the U.S. Pat. No. 5,644,342 wherein addressing of the various resistors is effected via three-dimensional (or 3-D) addressing.
The known head 10 (FIG. 1) comprises a plurality of resistors 11 each connected to a power transistor 12. The activation of the resistors 11 is controlled by means of an external control circuit connected to the head 10 itself through two corresponding flexible circuits (head flat cable and printer flat cable) having a plurality of contact points AI=1xe2x88x92N, PJ=1xe2x88x92M, GNDK=1xe2x88x92L 
where:
N is the number of resistors 11 or addresses AI that can be selected inside an activating group 14;
M is the number of pairs of primitives PJ that can be selected; and
L is the number of ground contacts GNDK or columns that can be selected.
For example, a head of 624 nozzles can be produced, according to the known art, from an integrated circuit comprising 24 pairs (M) of activating groups 14 with 13 (N) resistors 11 and associated transistors 12.
Accordingly:
M+N+L=39 contacts allows the external circuit to control the selective activation of
24*13*2=624 nozzles
by activating the ground connection of a column of primitives through a ground contact GNDK=1xe2x88x92L (Ground Select);
by activating, for a determined time and by means of the transistors 12, a first address AI (Address Line Select);
by electrically powering, within the column activated, with predefined current pulses and by means of the contacts PJ (Primitive Select), a predefined configuration of primitives corresponding to the first address AI;
by activating in sequence a second address AI (Address Line Select) and the relative transistors 12;
by electrically powering, within the column activated, with predefined current pulses and by means of the contacts PJ (Primitive Select), a second predefined configuration of primitives corresponding to the second address AI;
and so on in successive steps until activation of the N addresses is complete, and then continuing in similar fashion after de-activating the ground connection of the first column of primitives and activating that of the second column of primitives.
As is known, with 3-D addressing it is possible to limit the number of contact points between head and control circuit when the number of nozzles is very high. This is very important since, as is known, integrated heads are economically advantageous and reliable only if the number of contacts is limited to 50-60; indeed, it is only by limiting the number of contacts that it is possible to limit the surface of the integrated circuit constituting the head, the surface of the head flat cable and/or of the head flat cable/printer flat cable area of contact.
For example, as will be apparent to those acquainted with the sector art, the head with 624 nozzles of FIG. 1, if made with 2-D addressing, would not be economically advantageous as it would require:
2*24 (M)+13 (N)+24 (GND)=85 contacts.
But 3-D form addressing made in the known way of FIG. 1 presents a certain number of technical problems compromising its reliability and use possibilities.
A first problem consists of the fact that the intensity of current in the contact points GNDK=1xe2x88x92L between head flat cable and printer flat cable, being directly proportional to the primitives PJ powered simultaneously, may assume extremely variable values, for example between 250 mA and 6 Ampere (24*250 mA), assuming naturally that the minimum activating current of a resistor 11 is 250 mA. As will be obvious to those acquainted with the sector art, head flat cable/printer flat cable pressure contacts with these current values are not reliable or repeatable over time.
A second technical problem lies in the fact that the switches in the external control circuit for the ground contacts GNDK must be over-sized so as to be capable of supporting, for example, peak current values of magnitude 6 Ampere with very low resistances for closing the switches in question.
A third technical problem lies in the fact that the layout of this 3D type integrated head is very critical with regard to the ground conductors, which must support variable and very high peak currents in the head flat cable/printer flat cable contacts, and which have high parasitic resistances, with variable paths and high and variable voltage drops.
A fourth technical problem lies in the fact that the head flat cable/printer flat cable pressure contacts, with regard in particular to the contact points GNDK=Ixe2x88x92L, may cause sparks and electric discharges in the presence of high peak currents.
In short, the known 3-D integrated heads, due to the grouping of the ground connections, imply technical problems that are difficult to solve and signify poor reliability, production difficulties and variability of the power supplied to the resistors 11 for activating the emission of ink from the nozzles.
The object of this invention is to overcome the technical problems outlined above with an innovative integrated head layout which, at the same time, has the advantages of 3-D addressing and, unlike the known art, is easy to produce and reliable.
This object is attained by the integrated printhead characterised by logic means associated with each group of ink jet elements and with the second addressing means XK and suitable for being activated by logic signals for selecting one of the L columns of the array of groups of ink jet elements.
According to another characteristic of this invention, the integrated printhead, to advantage, may be driven by the control circuit via a four-dimensional, or 4-D, form of addressing.